1. Field of the Invention
This invention relates generally to the field of seismic prospecting and more particularly to a method and system for aligning the shooting paths of impact-type surface seismic sources.
2. Description of the Prior Art
An impact-type seismic source of the type described in U.S. Pat. No. 4,284,165, entitled "Acoustic Pulse Generator", issued Aug. 18, 1981 to Tom P. Airhart et al, herein incorporated by reference, is used to generate and record compressional waves, commonly termed P-waves. In such a device, in order to get proper results, the longitudinal axis of the cylindrical actuator housing along which an impact mass is propelled toward an earth-contacting base plate must be perpendicular to the plane of the bottom surface of such base plate in contact with the earth. This is true for non-horizontal as well as horizontal surfaces. If such perpendicularity is not maintained a significant portion of the available energy will be expended in producing shear waves of SH or SV type. In a P-wave generator, this would be inefficient and could complicate the task of seismic waves analysis and interpretation.
Conversely, one may desire to create and record a seismic pulse which possesses components of both P-waves and shear waves, either SH or SV type, in certain proportion depending upon the subsurface formation characteristics under investigation. An impact-type seismic generator of the type described above whose impact mass is propelled to its target along an inclined shooting path of variable direction is described in commonly-assigned co-pending U.S. patent application Ser. No. 676762, filed Nov. 3, 1984, in the name of Tom P. Airhart and entitled "Mounting and Control Means for Full Wave Form Seismic Source". In a device as described is such application the actuator housing is supported within a pair of "gimbal" type frames carried on a transport vehicle. These frames provide two degrees of rotational freedom for the actuator. Thus, the actuator may rotate within a first frame about a first axis transverse to the longitudinal vehicle axis. The first frame together with the first axis is in turn rotatable within a second frame about a second axis perpendicular to the first and extending along the longitudinal vehicle axis. The second frame is fixed to the vehicle chassis. By rotating the actuator about both these "forward-back" and "left-right" axes, it is possible to make the axis of the actuator housing, and hence the "shooting path" of the impact mass, coincide with any prescribed direction in space. The wave content of any seismic wave generated and recorded with such a device depends upon (1) the azimuth of the shooting path followed by the impact mass with respect to a line between the source and the recording geophones (the "seismic line") and (2) the angle of inclination of such path referenced, for the reasons explained above, to a perpendicular to the plane of the base plate. Hereafter in the detailed description to follow, the term "inclination" as applied to such a path shall be understood to mean the angle it makes with such perpendicular.
In seismic prospecting it is frequently desirable to simultaneously operate multiple vehicle-transported sources offset at about the same distance from an array of series of arrays of recording geophones. In order that the resultant seismic wave forms reinforce each other for maximum amplification, all such source vehicles must be headed in the direction of the seismic line or at the same angle to such line. The reason is that heretofore shooting paths and adjustments in direction have always been referenced to vehicle heading. This is sometimes difficult if not impossible to achieve with any degree of reliability. Visual sighting between any vehicle and the nearest geophone array or between vehicles may be hindered by terrain, vegetation or weather. Consequently, individual vehicles may be randomly misaligned with each other and with the seismic line. This is particularly troublesome when inclined impacts are programmed, because of the directionality of the energy distribution of SH and SV type shear waves produced with such impacts. If individual source vehicles have different headings and all actuators are rotated on the above-described gimbal frames through the same "forward-back" and "left-right" angles to reach the prescribed direction, the resulting paths will clearly differ from each other. Therefore, the wave forms recorded along the seismic line will also differ in content as to compressional and shear wave components. Vertical summation of these individual recordings will necessarily introduce distortion and degrade the quality of the seismic data.